1. Technical Field
The present technology pertains generally to electronic gas sensors and more particularly to chemical sensitive field effect transistor (CS-FET) devices that can be used as low energy consumption, highly sensitive, small size, multi-gas detecting chemical sensors alone or in an array of sensors.
2. Background
Microelectronic chemical sensors have been explored as low-cost alternatives to gas chromatography and other complex and expensive laboratory sensing devices since the 1970's. One major class of chemical sensor technologies is based on a change in conductivity of a film of a chemically sensitive material such as a metal oxide or a polymer.
Most commercial gas sensors are based on thick ceramic films made of polycrystalline post transition metal oxide semiconductors such as SnO2, ZnO and In2O3. The two types of metal oxide sensors are the n-type, which responds to oxidizing gases, and the p-type, which responds to reducing gases. For example, p-type sensors respond to oxidizing gases like O2, NO2, and Cl2 as these gases remove electrons and produce holes, i.e. produce charge carriers. By comparison, the n-type sensor operates by reacting oxygen in the air with the surface of the sensor that traps any free electrons on the surface producing resistance in these areas. However, if the sensor is exposed to a reducing gas the resistance drops because the gas reacts with the oxygen and releases an electron.
The presence of gases exposed to the sensor is detected by monitoring the resistance of the oxide layer of the sensor. Incoming gas molecules react with pre-adsorbed oxygen, hydroxyl or water molecules resulting in an increase of the resistance for oxidizing gases such as NO2 and O3 and a decrease for reducing gases such as H2, H2S, CH4, NH3, CO and NO. The change of conductance is usually proportional to the concentration of the detected gas in the feed. The sensitivity is also dependent on the electronic properties of the oxide material as well as its porous microstructure.
The first reason that traditional oxide gas sensors require elevated operating temperatures is that the surface oxidation and reduction reactions that are necessary for both sensor response and recovery are too slow at temperatures below 200° C. Conventional metal oxide gas sensors reach higher selectivity by changing the chemical state of chemisorbed oxygen at the surface from O2− to the more reactive O− by operating the sensor at temperatures typically between 300° C. and 600° C. At these elevated temperatures, certain feed gas molecules, such as hydrogen, methane, carbon monoxide, or hydrogen sulfide that can be chemically reduced by the oxygen species that are present at the surface of the metal oxide film.
The second reason for elevated operating temperatures is that adsorbed water on the metal oxide surface inhibits the sensor-analyte gas reactions, and therefore the water must be removed by operating at temperatures above 100° C., the boiling point of water. Desorption of adsorbed water molecules at high temperature therefore improves sensitivity further.
The consequence of these high temperature operational requirements is that the sensor must feature a heating unit to provide the required temperatures and therefore the power consumption of MOS based devices is very high compared to sensors fabricated from other materials. Power requirements also limit the portability of traditional MOS sensor systems.
Another important disadvantage of traditional oxide sensors is their lack of selectivity. Metal oxide-based gas sensors are often sensitive to more than one chemical species in a sample of feed gases and usually show cross-sensitivities. Consequently, the sensor measurements or signal from a target gas may be distorted or diminished by the interfering signal from the cross-reacting gases.
Accordingly, there is a need for gas sensing devices that can be operated at room temperature resulting in much lower power consumption and providing an ideal platform for portable devices. There is also a need for materials and methods that provide effective sensing at low cost. The present invention satisfies these needs as well as others and is generally an improvement over the art.